Water will be wet steam in which a steam gas (a gas phase portion: saturated steam) and water drops (a liquid phase portion: saturated water) are mixed after reaching a boiling point. Here, a weight ratio of the steam gas with respect to wet steam is called “dryness”. For example, if there exist the steam gas and the water drops in a ratio of 1:1, the dryness will be 0.5. In a case where there is no water drops and only the steam gas exists, the dryness will be 1.0. It is required to set the dryness of wet steam is close to 1.0 from the viewpoint of using sensible heat and latent heat possessed by wet steam effectively in a heat exchanger and so on, or from the viewpoint of preventing corrosion of turbine blades in a steam turbine. Accordingly, various methods for measuring the dryness have been proposed.
For example, the invention described in Patent Literature 1 relates to a technique of calculating the dryness by calculating a saturated water enthalpy and a saturated steam enthalpy using a saturated steam table based on wet steam flow rates and pressures on the upstream side and the downstream side of a pressure regulating valve by using a fact that the total enthalpy does not change on the upstream side and the downstream side of the pressure regulating valve provided in a pipe.
The invention described in Patent Literature 2 relates to a dryness measurement device including, for measuring the dryness quickly, (a) a light emitting body for irradiating wet steam with light, (b) a photo-receiving element for receiving light transmitted through the wet steam, (c) an environment sensor for measuring a temperature or a pressure of the wet steam, (d) a relationship storing portion for storing the relationship between the intensity of light transmitted through the wet steam and the dryness of the wet steam for each temperature or pressure and (e) a dryness specifying portion for specifying a value of the dryness of wet steam based on the measurement value of the intensity of light in the photo-receiving element, the measurement value of the temperature or the pressure by the environment sensor and the above relationship.